This invention relates generally to drilling for oil and gas from a floating drilling rig.
In marine applications, the marine riser connects a rig on the water's surface to a well bore at the seabed. Drilling risers or production risers may be employed for offshore applications. A marine drilling riser is designed to facilitate a closed drilling fluid system, where fluid pumped down the drill pipe returns back to the drilling rig through a system from seabed to surface. Production risers, used in the completion phase of a subsea well, provide access to the well bore through the completion tubing deep into the well with return of the completion fluids or mud remaining within a closed pumping system from the deepest circulating position through the seabed to surface.
Generally, risers undergo severe tensile, bending and/or torsional loading while in operation. Currently and conventionally, risers are made of very high strength steel.
Tension is applied from the floating rig to the riser through the riser tensioner system on the drilling rig. The amount of applied tension is a function of the depth of water between the rig and the seabed floor. The deeper the drilling operation, the longer the riser that must be provided. The longer the riser, the greater the weight and the required tension that is applied to the floating drilling rig. Thus, the deck load on the rig and the applied tension are determined by the length of the riser which, in turn, is determined by the depth of the water over the drilling operation.
Risers are assembled in sections called joints which vary in length, but generally are about 70–75 feet long. While casing or drill pipe joints are threaded and screwed together, riser joints are connected using a high pressure flanges and bolts. Usually steel or titanium connectors or end fittings are used, especially so in deep water drilling operations. The joints need to absorb not only axial loads but also bending stresses and high vibrations that exist in hostile marine environments.
The use of composite materials to reduce the weight of risers has been proposed for some time. However, the use of composites has not been well received in the industry for a variety of reasons. One of those reasons is the need to bond the composite material to the metal connectors while maintaining the mechanical integrity of tensile steels or equivalent. Thus, a critical composite riser design element is the load transfer mechanisms between the integrated steel/composite tube body and the end fittings.